chore: remove 3 lines of unused code in pkg/protocol/request.go

[xiaosau]

Change-Id: Id7bf1485b0aefa27826043ab90e9558ec6feffa6

What type of PR is this?

chore: remove 3 lines of unused code in pkg/protocol/request.go

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(Optional) More detailed description for this PR(en: English/zh: Chinese).

zh(optional): 删除pkg/protocol/request.go下三行无用代码：
Go 标准库的 multipart.ReadForm 即使没有 file part 也会返回 File: map[string][]*FileHeader{} （非 nil map），即使返回nil map，后续直接读也会返回nil

// 标准库实现
func (r *Reader) readForm(maxMemory int64) (_ *Form, err error) {
	form := &Form{make(map[string][]string), make(map[string][]*FileHeader)}
        / .....
}

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